1. Technical Field
The present invention relates to an improved flexible package with walls made up of multi-layer thin films. In particular, the package comprises a wall with a permeability characteristic that reduces absorption of substances associated with a packaged food product, and thereby prevents swelling and disfiguration of the package wall.
2. Description of Related Art
Packages made up of multi-layer flexible thin films are commonly used as food containers. The materials used to construct such films are generally chosen for their barrier properties, which act to prevent shelf life-reducing agents, such as oxygen, moisture, and light, from entering the bag. Different materials have different resistances to penetration of oxygen and moisture, so such variations are factors considered when the combination of films that make up a package is selected. It is also generally true that the cost of flexible films, and the containers they form, increases as the films"" barrier characteristics improve. So, from a manufacturing perspective, it is desirable to maximize barrier characteristics, while minimizing cost.
As newer materials with improved barrier properties are developed, and package designs are improved to incorporate those developments, the shelf life of the food product in the improved package generally increases. One example of such an improvement is the use of ethyl vinyl alcohol (EVOH) as one of the layers making up a food container. Use of EVOH in food packages makes them more resistant to the penetration of oxygen, which in turn increases the shelf life of the product.
The various film layers incorporated into flexible food packages are often employed for purposes other than to prevent penetration of agents that directly affect product shelf life. For example, a layer may be incorporated to preserve another layer of the package wall. In this regard, ethyl vinyl alcohol (EVOH), an excellent barrier to oxygen, is often used as one of the layers making up a food package. However, its resistance to oxygen penetration is reduced if it is exposed to moisture. Additionally, moisture that could erode the oxygen resistance of EVOH is often present not only on the outside of a package, but, depending on the type of food product, may also be present on the product side of the package wall. Therefore, the package wall must often include barriers to moisture on both the outside and product side of an EVOH layer.
In addition to moisture, any oils and other substances associated with a food product also come in contact with the product side of a package wall. For example, FIG. 1 is a cross-sectional view of an embodiment of the wall of a typical multi-layer prior art food package. The first flexible thin film 220 is on the product side 210 of the package wall 200, and comprises a sealant film layer 222 and a core film layer 224. Preferably, the core film layer 224 is a polyolefin film, such as a polypropylene (PP) or polyethylene terephthalate (PET), or metalized PP or metalized PET. The sealant layer 222 typically comprises a ter-polymer blend of polypropylene, polyethylene, and polybutene. The ratio of these polymers on a weight basis is typically 5% polyethylene, 2% polybutene, and the remainder polypropylene. The sealant layer 222 is on the product side 210 of the first film 220. Thus, the sealant layer 222 comes into contact with the food product in the package, which may be chips, candies, or other snacks. Any other substances associated with the food product, such as preservatives, flavoring, moisture, or oils, also come into contact with the sealant layer 222 of the first film 220.
The second film 230 typically includes an EVOH layer between protective polymer layers. Alternatively, the second film 230 may include weakly bonded polymer layers that prevent tearing of the package upon opening by the consumer. The third film 240 includes a PP or PET film layer 244 that is on the outside 250 of the package, and often includes a printed ink layer 242, the characteristics of which depend on the application. When a printed ink layer 242 is included, the PP or PET film layer 244 is made transparent so that the ink layer 242 can be viewed.
One unforeseen consequence of improvements in barrier properties and the resultant extensions of product shelf life is excessive absorption by the package wall of substances associated with the packaged product, such as oils or moisture. Such substances come into direct contact with the inner surface of the package wall throughout the useful life of the package. Prior art packages made up of multi-layer thin films designed to possess barrier properties that prolong a product""s shelf life did not anticipate absorption of oils from within the package that could become excessive and have destructive consequences. Over time, and especially at elevated temperatures, substances associated with a product that are in direct contact with the inner surface of a prior art package wall are absorbed by the thin films that make up the package wall. As a result, accumulation of the absorbed substances results in swelling of package walls, and blemishing of the package""s appearance.
FIG. 2 illustrates a cross-sectional view of the disfigured appearance of a prior art package wall 300 due to absorption of substances, such as oils and/or moisture associated with a food product inside the package. As in FIG. 1, the first flexible thin film 10 comprises a sealant film layer 12 and core film layer 14, which may be any of PP, PET, metalized PP or metalized PET film. In applications requiring a prohibitive oxygen barrier, a second flexible thin film 30 typically includes an EVOH layer 34 sandwiched between moisture-blocking polymer layers 32, 36. A third flexible thin film 40, also a PP or PET film, is on the outside 50 of the package. Any liquid or semi-solid substances, such as oils and/or moisture, that are associated with a packaged product on the inside 20 of the package wall 300 may be absorbed by the wall 300. In the case of oils, the oils penetrate the sealant layer 12 and accumulate in the first thin film 10. Over time, and especially at elevated temperatures, the accumulation in the first thin film 10 causes it to swell. The swelling 15 is manifested in the appearance of the package as the surface 302, 304 of the package wall 300, on both the inside and outside of the package, develops a slightly uneven look, which may be described as dimpled or puckered. This undesirable disfiguration greatly reduces the shelf appearance, and aesthetic market value, of the package.
Therefore, an improved package design and method are needed that minimize the absorption of product oils in the walls of a package containing food. The improvement should provide optimum shelf appearance for a longer time period. At the same time, the cost associated with the improved package should be minimized and other package characteristics, such as product shelf-life enhancing properties, should be maintained.
The proposed invention comprises a flexible food package formed from multi-layer thin films, where a film layer having a permeability characteristic that reduces absorption of food product substances from within the package is incorporated into the package wall design. The permeability characteristic, also referred to herein as xe2x80x9creduced permeabilityxe2x80x9d and xe2x80x9creduced permeability characteristic,xe2x80x9d is achieved by removing either the polyethylene or polybutene component from the ter-polymer sealant layer (making it a co-polymer layer) prior to orientation. The thus altered sealant layer decreases the amount of amorphousness of the polymer upon orientation, thereby reducing its permeability to food substances such as oil. An alternative embodiment of the invention reduces the amount of polyethylene and/or polybutene in the terpolymer of the sealant layer, leading as well to a decrease in amorphousness upon orientation and a reduction in permeability to oil. A reduction in permeability according to the present invention comprises reducing the amount of oil from a packaged food that is absorbed by the package wall, or slowing the rate at which oil from a packaged food is absorbed by the package wall. Maintaining either the polyethylene or polybutene in some amount in the sealant layer along with the polypropylene, however, maintains good sealing characteristics of the film.
Reducing absorption of food product oils by the package wall, or slowing the rate of absorption of such oils by the package wall, is increasingly necessary as better packaging technologies prolong the shelf life of food products. A longer shelf life lengthens the time the inner wall of the package is exposed to substances associated with the food product that the inner wall is likely to absorb from the food, such as oils and moisture. Through reducing absorption of product-side substances by the package wall over time, and especially at elevated temperatures, the invention prevents disfiguration of the package wall, such as swelling, and disfiguration of the package, such as puckering. The ability to better maintain the appearance of the package provides a significant advantage in a competitive food packaging environment.